1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a backlight unit for an LCD and a method for driving the same. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for increasing the life-span of a lamp of the backlight unit and reducing fabrication costs.
2. Discussion of the Related Art
Generally, a cathode ray tube (CRT) device, which is one type of flat display devices, has been widely used as monitors for a television, a measuring apparatus, and an information terminal. However, the CRT device does not have characteristics of compact size and lightweight. Thus, the display devices such as an LCD device using an electric field optical effect, a plasma display panel (PDP) using a gas discharge, and an electroluminescence display (ELD) device using an electric field luminous effect have been developed to substitute the CRT device. Among these display devices, since the LCD device is power-efficient, slim and lightweight, the LCD device has been most extensively researched and developed as monitors for desktop computers (or personal computers) and large sized display devices, as well as laptop computers (or notebook computers). Accordingly, the LCD devices have recently been in great demand.
Most LCD devices control the light transmittance from ambient light to display an image. In this respect, it is necessary to form an additional light source, such as a backlight unit, in an LCD panel. The backlight unit severs as a light source of the LCD device and is usually classified into a direct type and an edge type depending upon the location of a fluorescent lamp.
In the edge type backlight unit, a lamp unit is provided at a lateral side of a light-guiding plate. The lamp unit includes a fluorescent lamp, a lamp holder inserted to both ends of the fluorescent lamp to hold the fluorescent lamp, and a reflective sheet reflecting light emitted from the fluorescent lamp to the light-guiding plate. The edge type backlight unit is generally installed in small sized LCD devices such as monitors of laptop computers and desktop computers, since the edge type backlight is advantageous in a light uniformity, a long life-span, and a thin profile of the LCD device.
With increasing demands for large-sized LCD devices of 20-inch or more, the direct type backlight unit is actively developed, in which a plurality of lamps are arranged in one line on a lower surface of a light-diffusion sheet, so that the entire surface of the LCD panel is directly illuminated with light from the lamps. The direct type backlight unit has greater light efficiency than that of the edge-type backlight unit, and is suitable for the large-sized LCD device that requires high luminance. That is, the LCD device of the direct type backlight unit is generally installed in the large-sized monitor or the television. The large-sized monitor or the television is usually driven for a long time, and has a plurality of lamps, whereby it tends to have the lamps being turned off. In the direct type LCD device, a plurality of lamps are installed underneath the screen. Accordingly, if one of the lamps is turned off due to troubles of the lamp or the end of the lamp life, a screen portion corresponding to the turned-off lamp portion becomes darker as compared with surrounding portions of the screen. In this respect, it is necessary for the direct type LCD device to have a simple structure suitable for readily disassemble and assemble of the lamp unit.
Hereinafter, a related art backlight unit will be described with reference to the accompanying drawings. FIG. 1 is a perspective view of a related art direct type backlight unit for an LCD device. FIG. 2 is a schematic view of a fluorescent lamp and a connector of the related art direct type backlight unit in FIG. 1. FIG. 3 is a concept view schematically explaining a scanning driving method according to the related art.
As shown in FIG. 1, the related art backlight unit includes a plurality of fluorescent lamps 1, an outer case 3 for accommodating the plurality of fluorescent lamps 1, and light-scattering means 5a, 5b and 5c. The plurality of fluorescent lamps 1 are arranged at fixed intervals along one direction and fixed in the outer case 3. Then, the light-scattering means 5a, 5b and 5c are provided between the fluorescent lamps 1 and an LCD panel (not shown) to prevent a silhouette of the fluorescent lamps 1 from being reflected on a display surface of the LCD panel and to realize uniform luminance. To improve the light-scattering effect, a plurality of diffusion sheets and one diffusion plate are arranged between the fluorescent lamps 1 and the LCD panel. Also, a reflective sheet 7 is arranged inside the outer case 3 for concentrating the light emitted from the fluorescent lamps 1 to the display part of the LCD panel, thereby improving light efficiency.
Referring to FIG. 2, the fluorescent lamp 1 is a cold cathode fluorescent lamp (CCFL). Inside the fluorescent lamp 1, there are two electrodes 2 and 2a respectively mounted at two ends of the fluorescent lamp 1. As shown in FIG. 1, the both ends of the fluorescent lamp 1 are inserted into both sides of the outer case 3. Also, power supplying wires 9 and 9a are connected with the electrodes 2 and 2a of the fluorescent lamp 1 so as to provide the power for driving the fluorescent lamp 1. The power supplying wires 9 and 9a are also connected with a driving circuit by an additional connector 11. Thus, each fluorescent lamp 1 requires the additional connector 11. That is, the power supplying wire 9 connected to the electrode 2 and the power supplying wire 9a connected to the electrode 2a are both connected to the connector 11. Moreover, any one of the power supplying wires 9 and 9a is curved to the lower side of the outer case 3 for being connected with the connector 11.
However, in the related art backlight unit, the connector is connected with the power supplying wires of the fluorescent lamp, to be in contact with the driving circuit. Thus, each fluorescent lamp requires an individual connector. For this reason, the structure of the wires is complicated, and operation efficiency is lowered, because the power supplying wires are curved for being connected with the connector to decrease a thickness of the backlight unit. Also, it requires the additional process, thereby increasing a fabrication time and lowering a yield. In addition, a hole passing through the outer case is formed to connect the electrode with the connector, and the both electrodes of the fluorescent lamp are inserted to the hole to be exposed to the outside of the outer case. As a result, the operation efficiency is lowered, and maintenance and repair of the fluorescent lamp become difficult.
Further, unlike the CRT that is driven in a pulse type, the LCD device is driven in a hold type, so that the LCD device may have a problem of a motion blur generated due to overlapped images between frames on displaying a moving picture. In order to overcome this problem, as shown in FIG. 3, a backlight unit driven in a scanning method using CCFLs 30 is developed, which is an applied pulse type driving method of the CRT by sequentially turning on/off the plurality of CCFLs 30 by a video signal, thereby enhancing the characteristics of the moving picture.
However, as described above, when the backlight unit employs the CCFLs 30, each of the CCFLs 30 requires an individual inverter (31a, 31b, 31c, . . . , or 31n), thereby increasing fabrication costs. Also, since the CCFLs 30 are repetitively turned on/off, hydrargyrum (Hg) is exhausted due to a sputtering phenomenon caused in the internal electrode, thereby shortening the life-span of the CCFLs 30.